I’ve noticed that when I check the Date Modified for a folder and then open it, some files inside have a Date Modified later than the folder itself. This is confusing, because I sometimes sort folders by date only to find later that there're folders with stuff that was modified later. I'm using MacOS High Sierra.
Folders (on MacOS) are more generally called directories, and that's a better description. Each folder is a list of its contents. A folder is modified when the list changes, that is, when a item (a file or a sub-folder) is added, moved or deleted from the list.
Modifying a folder's files or sub-folders does not change the list, so changing a file does not change the folder's modification date. If a file or sub-folder is added or deleted from the folder, then the list is changed and the folder's modification date changes. That's why a folder's modification date is often older that the modification dates of its contents.
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